setListener method Null safety

void setListener (
  1. String name,
  2. ChannelCallback callback
)

Sets the listener for the specified channel.

When there is a listener, messages are sent immediately.

Each channel may have up to one listener set at a time. Setting a new listener on a channel with an existing listener clears the previous one.

Callbacks are invoked in their own stack frame and use the zone that was current when the callback was registered.

Draining

If any messages were queued before the listener is added, they are drained asynchronously after this method returns.

Each message is handled in its own microtask. No messages can be queued by plugins while the queue is being drained, but any microtasks queued by the handler itself will be processed before the next message is handled.

The draining stops if the listener is removed.

Implementation

void setListener(String name, ChannelCallback callback) {
  final _Channel channel = _channels.putIfAbsent(name, () => _Channel());
  channel.setListener(callback);
}